Piezoelectric thin film device

ABSTRACT

A piezoelectric thin film device according to the present invention comprises a lower electrode, a piezoelectric thin film and an upper electrode, in which the piezoelectric thin film is formed of an alkali niobium oxide-based perovskite material expressed by (K 1-x Na x )NbO 3  (0&lt;x&lt;1), and in which dependency of the piezoelectric constant d 31  of the piezoelectric thin film on applied electric field [=|(d 31  under 70 kV/cm)−(d 31  under 7 kV/cm)|/|d 31  under 70 kV/cm|] is 0.20 or less.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial no. 2008-013974 filed on Jan. 24, 2008, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to piezoelectric thin film devices using apiezoelectric thin film, more specifically to piezoelectric thin filmdevices including, on an Si (silicon) substrate, a piezoelectric thinfilm of an alkali niobium oxide-based perovskite material.

2. Description of Related Art

Piezoelectric materials are used for piezoelectric devices of variousapplications. For example, they are widely used for functionalelectronic components such as actuators in which an applied voltagedeforms a piezoelectric element thereby providing an actuation function,and sensors for detecting a physical quantity by utilizing, converselyto actuators, a voltage generated by a deformation of a piezoelectricelement. As piezoelectric materials for use in such actuators andsensors, there have been widely used lead-based dielectric materialswith excellent piezoelectric properties, in particular perovskitestructure ferroelectric materials expressed by the general chemicalformula: Pb(Zr_(1-x)Ti_(x))O₃ (often called PZTs). A PZT is typicallymade by sintering an oxide of its constituent metals.

In the trend toward downsizing and increasing performance of electroniccomponents, there is also a strong demand for piezoelectric devices withsmaller size and higher performance. However, as a piezoelectricmaterial made by widely used conventional sintering methods becomesthinner, the following problem comes to the fore. Specifically, as thethickness of a piezoelectric material approaches the order of 10 μm, itbecomes comparable to the grain size of the piezoelectric material;therefore, the influence of the grain boundaries can no longer beignored. This produces problems such as fluctuation in piezoelectricproperties and accelerated device degradation. In order to solve suchproblems by replacing conventional sintering methods, fabricationmethods of piezoelectric materials such as those utilizing thin filmformation techniques have been researched in recent years. Therefrom,there have been reported PZT films sputtered on an Si substrate for usein high-sensitivity gyro sensors (angular velocity sensors) (e.g., seeJP-A-2005-203725).

On the other hand, PZT piezoelectric sintered bulks and PZTpiezoelectric thin films contain approximately 60 to 70 mass % of lead;so, it is desired to promote research and development of lead-freepiezoelectric materials from an environmental consideration. Variouslead-free piezoelectric materials are currently being studied, amongwhich is potassium sodium niobate expressed by the general chemicalformula: (K_(1-x)Na_(x))NbO₃ (0<x<1) (hereinafter also referred to asKNN). A KNN has a perovskite structure and exhibits relatively excellentpiezoelectric properties among lead-free piezoelectric materials, and istherefore expected to be a promising lead-free piezoelectric materialcandidate. A KNN piezoelectric material has excellent piezoelectricproperties near x=0.5. And, there is a report that a KNN thin filmepitaxially formed on an MgO single crystalline substrate (instead of anSi substrate) exhibits good piezoelectric properties (see NonpatentDocument 1).

Nonpatent Document 1: T. Mino, S. Kuwajima, T. Suzuki, I. Kanno, H.Kotera, and K. Wasa: Jpn. J. Appl. Phys., 46 (2007) 6960.

Such KNN thin films have been attempted to be formed on an Si substrateby other film formation methods such as sputtering and PLD (pulsed laserdeposition). However, up to now, KNN thin films on an Si substrateexhibit a relatively low piezoelectric constant d₃₁ compared to PZT thinfilms, and therefore have yet to be applied to high-sensitivity sensorssuch as gyro sensors. Moreover, in the above Nonpatent Document 1, theKNN piezoelectric thin film is formed on an MgO substrate; however, useof MgO substrates presents a cost disadvantage since they are expensivecompared to silicon substrates.

SUMMARY OF THE INVENTION

Under these circumstances, the present invention addresses the aboveproblems. It is an objective of the present invention to provide apiezoelectric thin film device using a KNN thin film formed on an Sisubstrate which has sufficiently high performance to be applied to gyrosensors and the like.

In order to achieve the objective described above, the present inventionis configured as follows.

According to one aspect of the present invention, a piezoelectric thinfilm device comprises a lower electrode, a piezoelectric thin film andan upper electrode, in which the piezoelectric thin film is formed of analkali niobium oxide-based perovskite material expressed by(K_(1-x)Na_(x))NbO₃ (0<x<1), and in which dependency of thepiezoelectric constant d₃₁ of the piezoelectric thin film on appliedelectric field [=|(d₃₁ under 70 kV/cm)−(d₃₁ under 7 kV/cm)|/|d₃₁ under70 kV/cm|] is 0.20 or less.

In the above aspect of the present invention, the following improvementsand modifications can be made.

(i) The piezoelectric thin film has a strong (001)_(KNN) planediffraction peak which occupies 80% or more of diffraction peaks of thepiezoelectric thin film in an X-ray diffraction 2θ/θ measurement to asurface of the piezoelectric thin film.

(ii) The lower electrode is formed of a platinum (Pt) thin film.

(iii) Between the lower electrode and the piezoelectric thin film isinterposed a thin film of a material selected from a group consisting ofLaNiO₃, NaNbO₃ and (K_(1-x)Na_(x))NbO₃ (0<x<1) having a compositionratio x greater than that of the piezoelectric thin film.

ADVANTAGES OF THE INVENTION

According to the present invention, it is possible to provide apiezoelectric thin film device using a KNN thin film formed on an Sisubstrate which can greatly increase the piezoelectric constant d₃₁under lower applied electric fields by suppressing the dependency of thepiezoelectric constant d₃₁ on applied electric field to low levels, thusproviding sufficiently high performance to be applied to gyro sensorsand the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a cross-sectional view of apiezoelectric thin film device according to an embodiment of the presentinvention.

FIGS. 2( a) and 2(b) are schematic illustrations for explaining ameasurement method of the piezoelectric constant d₃₁ of a piezoelectricthin film device.

FIG. 3 is a schematic illustration showing a cross-sectional view of thepiezoelectric thin film device of Examples 1 to 4 and Comparativeexamples 1 to 6.

FIG. 4 is a table showing, for Examples 1 to 4 and Comparative examples1 to 6, forming condition of Pt/Ti film, occupation ratio of (001)_(KNN)plane, and piezoelectric properties of the KNN thin film.

FIG. 5 is an example of a diffraction pattern by an X-ray diffraction2θ/θ measurement to a surface of the piezoelectric thin film device ofComparative example 1.

FIG. 6 is an example of a diffraction pattern by an X-ray diffraction2θ/θ measurement to a surface of the piezoelectric thin film device ofExample 1.

FIG. 7 shows a relationship between piezoelectric constant d₃₁ andapplied electric field for piezoelectric thin film devices of Example 1to 4 and Comparative example 1 to 6.

FIG. 8 is a schematic illustration showing a cross-sectional view of thepiezoelectric thin film device of Examples 5 to 8.

FIG. 9 is a table showing, for Examples 5 to 8, type of an orientationcontrol layer, and occupation ratio of (001)_(KNN) plane andpiezoelectric properties of the KNN thin film.

FIG. 10 shows a relationship between piezoelectric constant d₃₁ andapplied electric field for piezoelectric thin film devices of Examples 5to 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described above, conventional KNN piezoelectric thin films on an Sisubstrate did not have sufficient piezoelectric constant d₃₁ to be usedfor gyro sensors. Typically, the piezoelectric constant d₃₁ of apiezoelectric thin film is relatively low under lower applied electricfields, while it is considerably higher under higher applied electricfields than under lower applied electric fields. That is, thepiezoelectric constant d₃₁ of a piezoelectric thin film generally has astrong dependency on applied electric field and tends to increase withincreasing applied electric field (i.e., the gradient of thepiezoelectric constant d₃₁ with respect to applied electric field ispositive and large).

Actually, the fact that piezoelectric thin films have low piezoelectricconstants d₃₁ under lower applied electric fields has been a limitingfactor in improving the sensitivity of gyro sensors. Moreover, it isconventionally regarded that such an above-described dependency of thepiezoelectric constant d₃₁ of a piezoelectric thin film on appliedelectric field is an essential phenomenon and therefore there is nosolution.

Through research and development of strongly preferentially (001)_(KNN)oriented KNN films on an Si substrate by the inventors, it has beenachieved a KNN piezoelectric thin film having a piezoelectric constantd₃₁ with a weak dependency on applied electric field. In further studyof thus obtained piezoelectric thin films, it has been revealed that, bysuppressing the dependency of the piezoelectric constant d₃₁ on appliedelectric field [=|(d₃₁ under 70 kV/cm)−(d₃₁ under 7 kV/cm)|/|d₃₁ under70 kV/cm|] to 0.20 or less, the piezoelectric constant d₃₁ (particularlythat under relatively low applied electric fields) can be greatlyincreased compared to those of conventional piezoelectric thin films(which will be detailed later with reference to e.g. FIG. 7).

By using such a lead-free KNN thin film on an Si substrate having suchexcellent characteristics, a piezoelectric thin film device can beprovided with sufficient properties to be applicable to gyro sensors orthe like, which conventional arts have had difficulty in providing. Inaddition, the use of an Si substrate enables the piezoelectric thin filmdevice of the present invention to be readily integrated withsemiconductor control circuits therefor or other semiconductor circuitsand devices on the same substrate.

A piezoelectric thin film device according to a preferred embodiment ofthe present invention will be described below with reference to theaccompanying drawings. However, the present invention is not limited tothe embodiments described herein.

FIG. 1 is a schematic illustration showing a cross-sectional view of apiezoelectric thin film device according to an embodiment of the presentinvention. As shown in FIG. 1, the piezoelectric thin film device 10 ofthis embodiment is fabricated by sequentially forming, on an Sisubstrate 1, a lower electrode 2, a KNN piezoelectric thin film 3 and anupper electrode 4.

The Si substrate 1 is an Si single crystalline substrate having a(100)_(Si) oriented surface (hereinafter “(100) Si substrate”). The Sisubstrate 1 may have an oxide film (SiO₂) formed on its surface in orderto electrically insulate the lower electrode 2 and Si substrate 1.

The lower electrode 2 serves as an important underlayer for forming theKNN piezoelectric thin film 3 thereon, and therefore it is preferable toemploy Pt (platinum) as the electrode material. This is because Pt filmsformed on the Si substrate 1 are self-oriented preferentially to a(111)_(Pt) plane. In this embodiment, the lower electrode 2 was formedof a Pt thin film grown by RF (radio frequency) magnetron sputtering. Inaddition, it is more preferable to provide a Ti (titanium) adhesivelayer between the Si substrate 1 and lower electrode 2 in order toenhance the adhesiveness of the lower electrode 2 (see FIG. 3, detailsare described later).

Unlike the lower electrode 2, the upper electrode 4, which is formed onthe KNN piezoelectric thin film 3, does not affect qualities of thepiezoelectric film 3. Therefore, there is no particular limitation onthe electrode material used. In this embodiment, similarly to the lowerelectrode 2, the upper electrode 4 was formed of a Pt thin film grown byRF magnetron sputtering.

The KNN piezoelectric thin film 3 is made of an alkali niobiumoxide-based perovskite material expressed by the general chemicalformula (K_(1-x)Na_(x))NbO₃ (0<x<1). Preferably, the composition x(=Na/[K+Na]) is approximately 0.5. The KNN piezoelectric thin film 3 canbe formed by sputtering, CVD, PLD, sol-gel process, etc. In thisembodiment, the KNN piezoelectric thin film 3 was formed by RF magnetronsputtering. And, occupation ratio of the (001)_(KNN) plane diffractionof the KNN piezoelectric thin film 3 is preferably 80% or more in anX-ray diffraction 2θ/θ measurement to a surface of the piezoelectricthin film 3. In addition, to the KNN piezoelectric thin film 3 of thisembodiment may be added any one of Ta, Li and Sb, or any combinationthereof.

Herein, an evaluation (measurement) method for a state of a crystalgrain alignment of the piezoelectric thin film by X-ray diffraction(XRD) is to be described. In an XRD 2θ/θ measurement, a specimen and adetector are scanned by the θ axis, wherein a scanning angle of thespecimen is θ and that of the detector is 2θ. According to the 2θ/θmeasurement, it can be estimated which crystal plane is a predominantplane at a surface of the piezoelectric thin film. Occupation ratio ofthe (001)_(KNN) plane of a KNN piezoelectric thin film is determinedusing diffraction peaks of KNN positioned at an angle 2θ between 20° and38° in the 2θ/θ measurement. Specifically, the occupation ratio of the(001)_(KNN) plane is defined as below:Occupation ratio of (001)_(KNN) plane (%)=[I _((001)KNN) /{I _((001)KNN)+I _((110)KNN)}]×100

in which

I_((001)KNN): diffraction peak intensity of (001)_(KNN) plane;

I_((110)KNN): diffraction peak intensity of (110)_(KNN) plane.

Here, the inventors consider that the X-ray diffraction peak positionedat an angle 2θ between 22.011° and 22.890° can be attributed to the(001)_(KNN) plane. Diffraction peaks due to the Si substrate and lowerelectrode are excluded from calculation of the occupation ratio of theKNN thin film. Also, it is in order to ensure the exclusion ofdiffraction peaks such as (002)_(KNN) plane and (111)_(Pt) plane fromthe calculation that the diffraction angle 2θ is limited to a rangebetween 20° and 38°. Furthermore, the X-ray diffraction in the presentinvention is always conducted by using the Cu—Kα ray.

On the other hand, the dependency of the piezoelectric constant d₃₁ ofthe KNN piezoelectric thin film 3 on applied electric field (kV/cm) isdefined by the expression: |(d₃₁ under 70 kV/cm)−(d₃₁ under 7kV/cm)|/|d₃₁ under 70 kV/cm|, i.e., absolute value of difference between“d₃₁ under 70 kV/cm” and “d₃₁ under 7 kV/cm” is divided by absolutevalue of “d₃₁ under 70 kV/cm”. The piezoelectric thin film 3 is formedso that this dependency value is 0.20 or less.

The piezoelectric constant d₃₁ of the piezoelectric thin film 3 will benow described with reference to FIGS. 2( a) and 2(b). FIGS. 2( a) and2(b) are schematic illustrations for explaining a measurement method ofthe piezoelectric constant d₃₁ of a piezoelectric thin film device.

Firstly, a rectangular strip is cut from the Si substrate 1 in FIG. 1 tofabricate an elongated piezoelectric thin film device 10. Next, one endof the piezoelectric thin film device 10 is clamped with a clamp 20 andthe other end is open to configure a simplified unimorph cantilever(FIG. 2( a)). Then, the KNN piezoelectric thin film 3 is stretched orcompressed by applying a voltage between the upper electrode 4 and lowerelectrode 2, thereby causing the entire cantilever (piezoelectric thinfilm device 10) to bend. And, the displacement Δ in the verticaldirection (the thickness direction of the piezoelectric film 3) at theother end (open end) of the cantilever is measured using a laser Dopplerdisplacement meter 21 (FIG. 2( b)).

The piezoelectric constant d₃₁ is calculated from the displacement Δ,the cantilever length, the thicknesses and Young's moduli of thesubstrate 1 and piezoelectric thin film 3 and the applied electric field(=[applied voltage]/[film thickness]). For details on the d₃₁calculation formula, see reference: I. Kanno, H. Kotera, and K. Wasa:Measurement of transverse piezoelectric properties of PZT thin films,Sens. Actuators A 107 (2003) 68.

The dependency of the piezoelectric constant d₃₁ on applied electricfield is determined by varying the electric field applied to thepiezoelectric thin film 3 of the cantilever. That is, the dependency ofthe piezoelectric constant d₃₁ on applied electric field[=|(d₃₁ under 70kV/cm)−(d₃₁ under 7 kV/cm)|/|d₃₁ under 70 kV/cm|] can be calculatedusing the d₃₁ values under 70 kV/cm and 7 kV/cm.

By suppressing the dependency of the piezoelectric constant d₃₁ of a KNNthin film on applied electric field to 0.20 or less, the piezoelectricconstant d₃₁, particularly that under relatively low applied electricfields (e.g., 7 kV/cm), can be greatly increased (see FIGS. 4 and 5,details are described later) and, as a result, there can be realized agyro sensor sensitivity comparable to that of gyro sensors using aconventional PZT thin film.

Further, a piezoelectric KNN thin film having a smaller dependency ofthe piezoelectric constant d₃₁ on applied electric field offers anadvantage that, when used as an actuator, the input voltage (or theinput electric field) is nearly proportional to the displacement, andtherefore no additional control circuits are required. Such apiezoelectric KNN thin film has another advantage of havingpiezoelectric properties resistance to deterioration with age andtherefore a longer service life.

A KNN thin film having a smaller dependency of the piezoelectricconstant d₃₁ on applied electric field can be achieved by increasing a(001)_(KNN) plane orientation preference thereof. In order to suppressthe dependency of the piezoelectric constant d₃₁ on applied electricfield to 0.20 or less, the occupation ratio of (001)_(KNN) planediffraction of the KNN thin film in the XRD 2θ/θ measurement ispreferably 80% or more. A KNN piezoelectric thin film 3 with a stronger(001) orientation preference can be obtained by, for example, using ahighly preferentially (111)_(Pt) plane oriented Pt thin film as thelower electrode 2 underlying the KNN film 3. The highly preferentially(111)_(Pt) plane oriented Pt thin film can be achieved by, for example,making thinner a Ti adhesive layer formed between the Pt thin film andSi substrate, forming the Pt thin film at higher temperatures, orsputtering the Pt thin film in an ambient with lower O₂ partialpressure.

A KNN piezoelectric thin film 3 having a smaller dependency of thepiezoelectric constant d₃₁ on applied electric field can also beachieved by interposing, between the Pt lower electrode 2 and KNNpiezoelectric film 3, an orientation control layer such as a LaNiO₃ thinfilm, a NaNbO₃ thin film and an Na rich (K_(1-x)Na_(x))NbO₃ thin filmhaving a composition ratio x greater than that of the KNN piezoelectricthin film 3. The orientation control layer is for enhancing the(001)_(KNN) orientation preference of the KNN piezoelectric thin film 3formed on the Pt lower electrode 2. By forming such a film (e.g., aLaNiO₃ thin film) on the lower electrode 2, a KNN film formed thereoncan be made to exhibit a stronger (001)_(KNN) orientation preferencethan one formed directly on the Pt lower electrode 2.

Referring to FIG. 1 again, a sensor for detecting a physical quantitycan be obtained by at least connecting a voltage detecting means betweenthe lower electrode 2 and upper electrode 4. Deformation of thepiezoelectric thin film device of this sensor due to a change in somephysical quantity will generate a corresponding voltage; thus, variousphysical quantities can be detected by sensing such voltage. On theother hand, an actuator can be obtained by at least connecting a voltageapplying means between the lower electrode 2 and upper electrode 4 inFIG. 1. A voltage application to this sensor will deform thepiezoelectric thin film device, thereby enabling actuation of variousmembers. Such sensors include gyro sensors, supersonic sensors, pressuresensors, and velocity/acceleration sensors. And, such an actuator can beused, e.g., in inkjet printers, scanners and supersonic generators.

EXAMPLES

Examples of the invention will be described below, however the presentinvention is not limited by these examples.

Examples and Comparative Examples of Piezoelectric Thin Film DeviceHaving Structure in FIG. 3

FIG. 3 is a schematic illustration showing a cross-sectional view of thepiezoelectric thin film device of Examples 1 to 4 and Comparativeexamples 1 to 6. The piezoelectric thin film devices 30 of Examples 1 to4 and Comparative examples 1 to 6 were fabricated by sequentiallyforming, on an Si substrate 11 (having an SiO₂ film 15 on its surface),a Ti adhesive layer 16, a Pt lower electrode 12, a (K_(0.5)Na_(0.5))NbO₃piezoelectric thin film 13, and a Pt upper electrode 14.

Next, the fabrication method of the piezoelectric thin film device ofExamples 1 to 4 will be detailed.

As the Si substrate 11, there was used an Si substrate with a thermaloxide layer (an SiO₂ film 15) on the substrate surface ((100)_(Si)single crystalline substrate of 4-inch round wafer, substrate thicknessof 0.5 mm, SiO₂ layer thickness of 0.5 μm). Firstly, on the Si substrate11 was sequentially formed the Ti adhesive layer 16 (thickness of 1 to 3nm) and the Pt lower electrode 12 (exclusively (111)_(Pt) oriented,thickness of 0.2 μm) by RF magnetron sputtering. The condition offormation of the Ti adhesive layer 16 and Pt lower electrode 12 was asfollows: substrate temperature of 350 to 400° C.; discharge power of 200W; introduced gas of Ar/O₂ (Ar/O₂=99/1 to 100/0); pressure of 2.5 Pa;and formation time of 1 to 3 min (for the Ti layer 16) and of 10 min(for the Pt electrode 12).

Subsequently, on the Pt lower electrode 12 was formed a 3-μm-thick(K_(0.5)Na_(0.5))NbO₃ piezoelectric thin film 13 by RF magnetronsputtering. The condition of formation of the (K_(0.5)Na_(0.5))NbO₃piezoelectric thin film 13 was as follows: sputtering target of sintered(K, Na)NbO₃ [composition ratio: (K+Na)/Nb=1, Na/(K+Na)=0.5]; substratetemperature of 600° C.; discharge power of 100 W; introduced gas of Ar;pressure of 0.4 Pa; and film formation time of 4 h.

On the (K_(0.5)Na_(0.5))NbO₃ piezoelectric thin film 13 was furtherformed a 0.02-μm-thick Pt upper electrode 14 by RF magnetron sputtering.The condition of formation of the Pt upper electrode 14 was as follows:without substrate heating; discharge power of 200 W; introduced gas ofAr; pressure of 2.5 Pa; and film formation time of 1 min.

Next, the fabrication method of the piezoelectric thin film device ofComparative examples 1 to 6 will be detailed.

As the Si substrate 11, there was used the same conditions' Si substratewith a thermal oxide layer 15 as was employed in Examples 1 to 4. First,on the Si substrate 11 was sequentially formed the Ti adhesive layer 16(thickness of 5 to 10 nm) and the Pt lower electrode 12 (exclusively(111)_(Pt) oriented, thickness of 0.2 μm) by RF magnetron sputtering.The formation conditions of the Ti adhesive layer 16 and Pt lowerelectrode 12 were as follows: substrate temperature of 250 to 350° C.;discharge power of 200 W; introduced gas of Ar/O₂ (Ar/O₂=90/10 to 98/2);pressure of 2.5 Pa; and formation time of 5 to 10 min (for the Ti layer16) and of 10 min (for the Pt electrode 12).

Subsequently, on the Pt lower electrode 12 was sequentially formed a3-μm-thick (K_(0.5)Na_(0.5))NbO₃ piezoelectric thin film 13 and a0.02-μm-thick Pt upper electrode 14 by RF magnetron sputtering. Theconditions of formation of the (K_(0.5)Na_(0.5))NbO₃ piezoelectric thinfilm 13 and Pt upper electrode 14 were the same as those used inExamples 1 to 4.

FIG. 4 is a table showing, for Examples 1 to 4 and Comparative examples1 to 6, forming condition of Pt/Ti film [(Pt lower electrode 12)/(Tiadhesive layer 16)], occupation ratio of (001)_(KNN) plane, andpiezoelectric properties of the KNN thin film. In Examples 1 to 4, inorder to cause the Pt thin film to be highly preferentially (111)_(Pt)oriented, the Ti adhesive layer was formed thinner (e.g., 1 to 3 nm) andthe Pt film was formed at a higher temperature (e.g., 350 to 400° C.)and a sputtering ambient having a lower O₂ concentration was employed(e.g., 0 to 1%) than in Comparative examples 1 to 6.

Further, in order to examine a state of a crystal grain alignment of the(K_(0.5)Na_(0.5))NbO₃ (KNN) piezoelectric thin films 13, an X-raydiffraction 2θ/θ measurement was performed for the piezoelectric thinfilm devices (of the above Examples and Comparative examples) whose Ptupper electrode 14 had not been formed and whose KNN thin film 13 wastherefore exposed. FIG. 5 is an example of a diffraction pattern by anX-ray diffraction 2θ/θ measurement to a surface of the piezoelectricthin film device of Comparative example 1; and FIG. 6 is an example of adiffraction pattern by the X-ray diffraction 2θ/θ measurement to asurface of the piezoelectric thin film device of Example 1. ComparingFIG. 5 with FIG. 6, it can be seen that the KNN thin film 13 of Example1 exhibits a considerably stronger (001)_(KNN) orientation preference (ahigher occupation ratio of (001)_(KNN) plane) than that of Comparativeexample 1.

In FIG. 4 is shown the occupation ratio of (001)_(KNN) plane of the KNNthin films of Examples 1 to 4 and Comparative examples 1 to 6. The KNNthin films of Examples 1 to 4 exhibit the occupation ratio of(001)_(KNN) plane of as high as 80% or more.

The piezoelectric thin film devices of the above Examples 1 to 4 andComparative examples 1 to 6 were measured for the piezoelectric constantd₃₁. The measurement was done according to the method described above inFIGS. 2( a) and 2(b). For use in the cantilever, a 20-mm-long and2.5-mm-wide rectangular strip of the piezoelectric thin film device wasfabricated. The piezoelectric constant d₃₁ was calculated using aYoung's modulus of 104 GPa for the KNN piezoelectric thin film 13.

FIG. 7 shows a relationship between piezoelectric constant d₃₁ andapplied electric field for piezoelectric thin film devices of Examples 1to 4 and Comparative examples 1 to 6. In FIG. 4 are also shown gradientof the piezoelectric constant d₃₁[=|(d₃₁ under 70 kV/cm)−(d₃₁ under 7kV/cm)|/|d₃₁ under 70 kV/cm|]; the d₃₁ value under 7 kV/cm; and the d₃₁value under 70 kV/cm as piezoelectric properties of the KNN thin film.

For example, the gradient of the piezoelectric constant d₃₁ of Example 1is calculated from: |(d₃₁ under 70 kV/cm)−(d₃₁ under 7 kV/cm)|/|d₃₁under 70 kV/cm|=|(−79 pm/V)−(−63 pm/V)|/|−79 pm/V|]=0.2.

FIGS. 4 and 7 also show the piezoelectric constant d₃₁ of a prior artobtained for a KNN film formed on an Si substrate (reported in: Y.Nakashima, W. Sakamoto, H. Maiwa, T. Shimura, and T. Yogo: Jpn. J. Appl.Phys., 46 (2007) L311). The above prior art forms the KNN film by CSD(chemical solution deposition) and probably provides the best datareported in papers for KNN films on Si substrates. In addition, the KNNfilm of the above prior art has about 50% (001)_(KNN) oriented grainsand about 50% (110)_(KNN) oriented grains. The above prior art reportsthat the d₃₃ value is 46 pm/V. In FIG. 4, the prior art d₃₁ value isdetermined to be −23 pm/V using the assumption that “d₃₁=−d₃₃/2”. As isapparent from Examples 1 to 4 in FIGS. 4 and 7, the piezoelectricconstant d₃₁, particularly that under relatively low applied electricfields (e.g., 7 kV/cm), can be greatly increased either by suppressingthe dependency of the piezoelectric constant d₃₁ on applied electricfield[=|(d₃₁ under 70 kV/cm)−(d₃₁ under 7 kV/cm)|/|d₃₁ under 70 kV/cm|]to 0.20 or less and/or by increasing the occupation ratio of (001)_(KNN)plane to 80% or more.

Examples of Piezoelectric Thin Film Device Having Structure in FIG. 8

Examples of the piezoelectric thin film device having a structure inFIG. 8 will be described below. FIG. 8 is a schematic illustrationshowing a cross-sectional view of the piezoelectric thin film device ofExamples 5 to 8. As shown in FIG. 8, the piezoelectric thin film device80 of Examples 5 to 8 was fabricated by sequentially forming, on an Sisubstrate 11 (having an SiO₂ film 15 on its surface), a Ti adhesivelayer 16, a Pt lower electrode 12, an orientation control layer 17, a(K_(0.5)Na_(0.5))NbO₃ piezoelectric thin film 13 and a Pt upperelectrode 14.

Next, the fabrication method of the piezoelectric thin film device ofExamples 5 to 8 will be described.

As the Si substrate 11, there was used the same conditions' Si substratewith a thermal oxide layer 15 as was employed in Examples 1 to 4.Firstly, on the Si substrate 11 was sequentially formed the Ti adhesivelayer 16 (thickness of 2 nm) and the Pt lower electrode 12 (exclusively(111)_(Pt) oriented, thickness of 0.2 μm) by RF magnetron sputtering.The formation conditions of the Ti adhesive layer 16 and Pt lowerelectrode 12 were as follows: substrate temperature of 400° C.;discharge power of 200 W; introduced gas of Ar; pressure of 2.5 Pa; andformation time of 2 min (for the Ti layer 16) and of 10 min (for the Ptelectrode 12).

Subsequently, on the Pt lower electrode 12 was formed an orientationcontrol layer 17 (thickness of 200 to 300 nm) by RF magnetronsputtering. As the orientation control layer 17, Examples 5 to 8respectively used a 200-nm-thick LaNiO₃ thin film, 300-nm-thick LaNiO₃thin film, 200-nm-thick NaNbO₃ thin film and 200-nm-thick(K_(0.2)Na_(0.8))NbO₃ thin film. The condition of formation of theorientation control layer 17 was as follows: sputtering target ofsintered LaNiO₃ (for Examples 5 and 6), sintered NaNbO₃ (for Example 7)and sintered (K_(0.2)Na_(0.8))NbO₃ (for Example 8); substratetemperature of 600° C.; discharge power of 100 W; introduced gas of Ar;and pressure of 0.4 Pa.

Then, on the orientation control layer 17 was sequentially formed a3-μm-thick (K_(0.5)Na_(0.5))NbO₃ thin film 13 and a 0.02-μm-thick Ptupper electrode 14 by RF magnetron sputtering. The formation conditionsof the (K_(0.5)Na_(0.5))NbO₃ thin film 13 and Pt upper electrode 14 werethe same as those employed in Examples 1 to 4.

Similarly to Examples 1 to 4, the occupation ratio of (001)_(KNN) planeof the KNN thin films 13 of Examples 5 to 8 were determined by the XRD2θ/θ measurement. Also, their piezoelectric constants d₃₁ were measuredusing the same method as used in Examples 1 to 4.

FIG. 9 is a table showing, for Examples 5 to 8, type of an orientationcontrol layer, and occupation ratio of (001)_(KNN) plane andpiezoelectric properties of the KNN thin film. Further, FIG. 10 shows arelationship between piezoelectric constant d₃₁ and applied electricfield for piezoelectric thin film devices of Examples 5 to 8.

As can be seen from Examples 5 to 8 in FIGS. 9 and 10, the interpositionof the orientation control layer 17 causes the occupation ratio of(001)_(KNN) plane to be as strong as 93% or more, and also furtherreduces the dependency of the piezoelectric constant d₃₁ on appliedelectric field[=|(d₃₁ under 70 kV/cm)−(d₃₁ under 7 kV/cm)|/|d₃₁ under 70kV/cm|] to as low as 0.11 or less while still exhibiting a high d₃₁value under 7 kV/cm.

Although the invention has been described with respect to the specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A piezoelectric thin film device, comprising: a lower electrode, apiezoelectric thin film and an upper electrode, wherein: thepiezoelectric thin film is formed of an alkali niobium oxide-basedperovskite material expressed by (K_(1-x)Na_(x))NbO₃ (0<x<1), andwherein: dependency of the piezoelectric constant d₃₁ of thepiezoelectric thin film on applied electric field [=|(d₃₁ under 70kV/cm)−(d₃₁ under 7 kV/cm)|/|d₃₁ under 70 kV/cm|] is 0.20 or less. 2.The piezoelectric thin film device according to claim 1, wherein: thepiezoelectric thin film has a strong (001)_(KNN) plane diffraction peakwhich occupies 80% or more of diffraction peaks of the piezoelectricthin film in an X-ray diffraction 2θ/θ measurement to a surface of thepiezoelectric thin film.
 3. The piezoelectric thin film device accordingto claim 1, wherein: the lower electrode is formed of a platinum thinfilm.
 4. The piezoelectric thin film device according to claim 1,wherein: between the lower electrode and the piezoelectric thin film isinterposed a thin film of a material selected from a group consisting ofLaNiO₃, NaNbO₃ and (K_(1-x)Na_(x))NbO₃ (0<x<1) having a compositionratio x greater than that of the piezoelectric thin film.